# Fundamentals of Computer Networks ECE 478/578

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Fundamentals of Computer Networks ECE 478/578
Lecture #8: Multiple Access Protocols Instructor: Loukas Lazos Dept of Electrical and Computer Engineering University of Arizona

The Channel Allocation Problem
How to share access to a common medium Attributes of the channel allocation problem Dynamic or Static Allocation A single channel is available Time is continuous vs. slotted Carrier sensing (CS) Collision detection (CD)

Multiple Access Protocols
Aloha (Pure vs. Slotted) Carrier Sense Multiple Access (CSMA) Collision Resolution Algorithms

Pure Aloha Continuous time, no CD, no CS

Assumptions of Pure Aloha
New arrivals of packets at each host are transmitted immediately Arrivals are Poisson with total rate λ If a packet is involved in a collision it is retransmitted after a random period of time. Node becomes backlogged Receivers provides feedback on received packet to implement the collision detection (we know a collision happened) No buffering: A backlogged node does not buffer any arriving packets Or the set of nodes accessing the medium is infinite

Collisions Occurence t t1 t2 t3 t4 t5 retransmission collision
Packets arrival: Poisson with rate G > λ packets/t. t t1 t2 t3 t4 t5 collision retransmission Vulnerable period: 2t

Aloha Throughput Probability that k frames are generated within the unit of time t Probability that no traffic is generated within 2t Aloha throughput: Arrival rate times success probability

Slotted Aloha t slot 1 slot 2 slot 3 slot 6 empty collision
Time is divided into slots Transmissions start only at the beginning of a slot A collision would occur in slot n, if more than one arrival occurs in slot n – 1 No arrival: Unused slot t slot 1 slot 2 slot 3 slot 6 empty collision retransmission

Throughput of Slotted Aloha
Simplistic Analysis: arrival rate at each slot is G Probability of successful transmission Probability of an idle slot Probability of a collision Probability of a wasted slot

Pure vs. Slotted Aloha

Carrier Sense Multiple Access (CSMA)
Stations listen before transmission 1-persistent Stations listen to the channel continuously If channel is busy wait till free If channel is free, transmit (i.e. with probability 1) If a collision occurs, wait a random amount of time

Alternative CSMA Strategies
Non-persistent CSMA If channel is busy, defer from sensing for a random time Better medium utilization than persistent CSMA p-persistent CSMA If channel is sensed busy, transmit when idle with probability p, else wait for a period τ. Low throughput on low loads due to idle time Improves throughput on high loads

Comparison of Protocols

CSMA with Collision Detection (CSMA/CD)
Detect a collision early and abort transmission Two types of slots, packet slots and contention slots Each node tries to transmit at a contention slot with probability p If collision, it is detected at the end of contention slot If success, no transmission tries till end of packet slot

Duration of Contention Slots
How long does it take for two stations to detect a collision? Α τ = d/c Β t0 = 0 Β Α τ - ε Β Α e 2τ - ε A will detect collision after 2τ in the worst case. Contention slots have to be at least 2τ long Minimum frame size 2τC (why?) Ex. 1Km cable, τ = 5μs, If C = 100MBps, Frame size 100bits C = 1 Gbps Frame size = 10,000 bits